The present invention relates generally to gas turbine engine airfoils having properties including improved stiffness and high cycle fatigue capability. More particularly, in one embodiment the present invention defines a single cast metallic airfoil structure having a high stiffness core positioned therein and metallurgically bonded thereto. The high stiffness core is preferably formed of gamma titanium aluminde or a titanium metal matrix composite. Although the present invention was developed for use in a gas turbine engine, certain applications may be outside of this field.
Modern gas turbine engines often utilize fan blades, compressor blades and vanes, and may operate at high rotation speeds and high compression ratios. Consequently, the rotating engine components must be strong, stiff, light in weight and high temperature resistant. Conventional airfoils in the fan and forward stages of the gas turbine engine compressor are typically machined from a titanium alloy forging. A titanium alloy is used because of its high specific strength. The stiffness of an airfoil is a function of airfoil geometry and the material modulus of elasticity. Since all of the titanium alloys used in gas turbine engine airfoil applications possess a similar modulus of elasticity, changes in airfoil stiffness have generally been accomplished by changes in geometry. In some instances, aerodynamic performance is compromised as a result of altering airfoil geometry in order to meet dynamic and high cycle fatigue requirements.
A conventional method to stiffen hollow titanium structures, such as fan blades, compressor blades and vanes include placing internal stiffening ribs or honeycomb structures within the hollow space of the structures. The stiffening ribs may be machined into the inner surfaces of the multiple bonded segments that comprise the structure, or the honeycomb may be inserted into the hollow space prior to bonding the multiple segments together.
An alternate approach for internally stiffening hollow components, such as fan blades, compressor blades and vanes, includes bonding a layer of high modulus metal matrix composite onto the inner surfaces of the structural segments comprising the component. The layer may comprise one or more piles of ceramic fibers and alloy matrix which form a stiffening layer of metal matrix composite on the inner surfaces of the structure. The metal matrix composite may be applied by methods such as vacuum hot pressing, hot isostatic pressing of alloys and fibers, or by plasma spray deposition of molten alloy powder over fiber mats. Such reinforced segments are then joined by conventional methods such as brazing, diffusion bonding or electron beam welding to form the desired structure.
Although the prior techniques of stiffening an airfoil are steps in the right direction, the need for additional improvement still remains. The present invention satisfies this need in a novel and unobvious way.